PROJECT SUMMARY/ABSTRACT The purpose of this proposal is to determine the efficacy of the combination of targeted therapy against MEK and VEGF in combination with immunotherapy in patients with refractory, metastatic colorectal cancer. The development of immunotherapy targeting immune regulatory checkpoints has started a new era in the treatment of cancer. Blockade of these immune checkpoints has shown tumor regression in several tumor types, however in colorectal cancer patients activity has been restricted to patients that have high microsatellite instability (MSI). However, a phase Ib study combining a MEK inhibitor, cobimetinib, and a PDL-1 inhibitor, atezolizumab, in patients with microsatellite stable metastatic colorectal cancer patients demonstrated a combination effect which provides support for combination strategies for these patients that are MSS Biopsies obtained from this trial will be used for two specific purposes; 1) evaluate immune and pathway modulation in patients with microsatellite stable metastatic colorectal cancer, and 2) the development of humanized patient-derived xenografts to conduct a co-preclinical trial. Our group, in collaboration with the immunology team at the University of Colorado Anschutz Medical Campus, has developed a humanized patient derived xenograft mouse model. In preliminary studies with this model we have observed anti-tumor activity in colorectal cancer models that are MSI-high demonstrating that the model has the potential to mimic what is seen in the clinical setting and to allow for preclinical studies of immune checkpoint inhibition. These data support our overall hypothesis that humanized colorectal PDX models can be utilized to evaluate combination strategies with immunotherapy and other targeted agents in MSS patients. We propose to 1) Identify immune and pathway modulation following treatment with binimetinib, bevacizumab, and pembrolizumab. 2) Determine the efficacy of the combination of binimetinib, bevacizumab, and immune checkpoint inhibitors in humanized patient derived xenograft models of microsatellite stable colorectal cancer, and 3) Identify immune and pathway modulation in humanized patient derived xenograft models treated with the combination.